In a nonvolatile semiconductor memory device typified by a NAND flash memory, while miniaturization is progressing, the element isolation region is configured to have a prescribed depth to ensure the electrical insulation between elements.
However, with the progress of miniaturization, the width of the floating gate becomes narrower. Hence, there is a problem that depletion of the upper portion of the floating gate is likely to occur and the stability of writing and the stability of reading are not sufficient. Thus, a nonvolatile semiconductor memory device is desired that is good in the stability of writing and the stability of reading even when miniaturization progresses.